1. Field of the Invention
The present invention relates to a composition used for lapping and to a lapping method, and, particularly, relates to a lapping composition containing milled powder of plate alumina for lapping metals or inorganic materials such as semiconductor monocrystals, various metals, glass plates, lens, quartz and so on, and a lapping method using this lapping composition.
2. Description of the Prior Art
"Lapping" is a grinding process which is performed on the surface of subject to be lapped (work), in advance to finishing into a mirror-like surface through "polishing".
Lapping process is usually performed in a manner such that a work is sandwiched between two, upper and lower cast ion plates, a water slurry of abrasives is fed between the surfaces of plates and the work, and the plates are rotated under pressure.
The purposes of this process are:
(1) to make the thickness uniform;
(2) to eliminate defects on the surface produced by cutting or sawing and to minimize the sub-surface damage layer (the layer in which minute cracks or pits are produced in the range of a certain depth from the surface);
(3) to obtain parallel, non-warp surfaces.
Depending on the material of the work, a suitable materials as the abrasive is selected. Usually, white alumina, brown alumina-zirconia composite, plate alumina, silicon carbide and so on are used.
Powders of white alumina, brown alumina-zirconia composite and silicon carbide are produced by milling the fused ingot of raw materials and classifying thereof, and the shapes of grains are not uniform. On the other hand, plate alumina powder is obtained by milling a monocrystal agglomerate obtained by calcinating aluminum hydroxide produced in a wet process at relatively low temperature. Grains thereof have a flat-plate like shape of hexagonal monocrystal so that it is called plate alumina.
The performance of the abrasive powder used in the lapping process is characterized not only in its hardness, fexibility and grain shape, but also in grain size and distribution thereof. Among them, the hardness and flexibility are attributes depending on the kind of the material.
The influence of grain shape onto the lapping action is important. It is said that grains having non-spherical shape but flat shape with edges have a large grinding efficiency (removal rate). In this sense, plate alumina is generally larger in removal rate than milled powder of fused ingot.
Grain size is also one of the factors having the largest influence onto the performance of abrasive powder. If grain size is large, the removal rate becomes large, but the energy of abrasive grains given onto the surface becomes large. Accordingly, there occurs a problem that defects produced at the time of lapping are increased and the sub-surface damage layer becomes deeper. On the contrary, if grain size is too small, the removal rate is too small to use.
As described above, in the case of the same material, the removal rate and the quality of the surface (defects, depth of sub-surface damage and so on) almost depend on the grain shape and grain size, but the removal rate and the surface quality are mutually inconsistent factors. It is therefore necessary to seek the most suitable grain shape and size so as to provide a good balance therebetween.
Further, the commercially produced abrasive powder in practice has size distribution so that it includes from fine grains to coarse grains. Therefore, the size distribution is also an important problem. Grains having too large size must be restricted severely because they cause scratches. In addition, fine grains which do not contribute to grinding are not only unnecessary, but also harmful in some cases.
U.S. Pat. No. 5,277,702 discloses plate alumina powder having a sharp distribution suitable to use mainly in lapping a silicon wafer, and discloses a method of producing such plate alumina powder. In this U.S. Pat., it is described that the production of fine grains; in milling is suppressed by improving the method of milling to thereby obtain powder having only a small number of fine grains or crushed pieces of primary grains. It is described that the reduction of fine grains decreases production of the defects or sub-surface damages in the lapping process. It is further described that the reason why the reduction of fine grains suppresses the defects or sub-surface damages in the lapping process is considered that in the case of plate alumina, the alumina grains flow in parallel with the lapped surface in the flow of the slurry, and if a lot of fine grains coexist, the fine grains lift up the edges of the plates to make them unbalanced so as to cause scratches, while the reduction of fine grains makes the flow of plates stable in the slurry to thereby make it possible to obtain a superior surface.
In the conventional lapping abrasives, efforts have been made in improving the raw materials, the method of milling, the method of classification so as to try to adjust the shape, average size and size distribution of the grains thereby to improve the performance. However, it is the fact that it is difficult to obtain a lapping abrasives which is satisfactory in balance between the removal rate and the quality of the surface.
Although the balance between the rate and the quality can be improved if the size distribution is made sharp so as to reduce the fine grains by repeating the number of times of the classification, the cost of production becomes higher correspondingly.